The present invention relates to a control apparatus for a multi-cylinder internal combustion engine which controls engine operation such as ignition timing, fuel injection timing and the like based on a reference position signal generated in synchronism with the rotation of a crankshaft of the engine. More particularly, it relates to an engine control apparatus in which reference crank positions for each cylinder can be determined in a precise manner.
In general, in a multi-cylinder internal combustion engine having a crankshaft driven by a plurality of cylinders and a camshaft operatively connected with the crankshaft, a plurality of reference position signals, which are generated by a reference signal generator in synchronism with the rotation of the crankshaft, are used for controlling engine operation such as ignition timing, fuel injection timing for each cylinder. Each of the reference position signals corresponds to a predetermined rotational angle of the crankshaft, which is hereinafter referred to as a predetermined crank angle or position. The reference signal generator is generally mounted on the crankshaft or the camshaft which is operatively connected with the crankshaft in synchronized rotation therewith.
FIG. 12 illustrates a typical example of a conventional engine control apparatus for a multi-cylinder internal combustion engine. The apparatus illustrated includes a first or cylinder identifying signal generator 1 mounted on an unillustrated camshaft, which is operatively connected to an unillustrated crankshaft of the engine for synchronized rotation therewith, for generating a cylinder identifying signal SC, and a second or reference signal generator 2 mounted on the camshaft for generating a reference position signal ST indicative of two predetermined reference positions corresponding to two predetermined crank angles. The cylinder identifying signal SC from the first signal generator 1 and the reference signal ST from the second signal generator 2 are supplied through a first and a second interface 3, 4, respectively, to a control unit in the form of a microcomputer 5 which controls the ignition timing for each cylinder based on these signals.
Generally, the camshaft, on which the cylinder identifying signal generator 1 and the reference signal generator 2 are mounted, is operatively connected with the crankshaft such that it performs one complete revolution per two revolutions of the crankshaft.
As shown in FIG. 13, the cylinder identifying signal generator 1 generate a cylinder identifying signal SC including an appropriate number of pulses each corresponding to a specific cylinder per one camshaft revolution, and the reference signal generator 2 generates a reference position signal ST including a plurality of reference pulses each corresponding to predetermined reference crank positions of a corresponding cylinder. For example, these signal generators 1, 2 may be constructed as follows. A rotating disk is mounted on the camshaft for integral rotation therewith and has a plurality of first and second arcuate slits formed therethrough. The first slits correspond in number to the cylinders and are disposed around the center of rotation of the disk at equal circumferential intervals. Each of the first slits has a leading edge and a trailing edge corresponding to two predetermined reference crank positions or angles for a corresponding cylinder. Each of the second slits corresponds to a specific cylinder. The first and second slits during the rotation of the disk are sensed by an appropriate sensing means such as a photocoupler which generates a cylinder identifying signal each time it senses one of the second slits, and a reference position signal each time it senses one of the first slits.
FIG. 13 illustrates a waveform diagram showing the waveforms of an example of the cylinder identifying signal SC and an example of the reference position signal ST, which are adapted for use with an engine having four cylinders #1 through #4. Here, the cylinder identifying signal SC includes two kinds of rectangular-shaped pulses generated for two specific cylinders, cylinder #1 and cylinder #4. The reference position signal ST includes a series of rectangular-shaped pulses each having a leading or rising edge corresponding to a second reference position of a corresponding cylinder, e.g., 75 degrees (B75.degree.) before top dead center (BTDC), and a trailing or falling edge corresponding to a first reference position thereof, e.g., 5 degrees (B5.degree.) BTDC.
The operation of the above-mentioned conventional engine control apparatus will now be described below while referring to the waveform diagram of FIG. 13. As the engine starts to operate, the first and second signal generators 1, 2 generate a cylinder identifying signal SC and a reference position signal ST which are fed to the microcomputer 5. Based on these signals SC and ST, the microcomputer 5 senses the first reference position B5.degree. and the second reference position B75.degree. of each cylinder and controls the optimum ignition timing and the optimum fuel injection timing for each cylinder in a timer-controlled manner on the basis of the first and second reference positions thus sensed while reflecting the running conditions of the engine such as the rotational number (rpm), the engine load, etc.
Specifically, during the low-speed or low-load operation of the engine, the ignition timing is electronically controlled in an ignition-retarding direction, whereas during the high-speed or high-load operation, the ignition timing is electronically controlled in an ignition advancing direction. The actual ignition timing is controlled through a timer with a length of time measured from the first or second reference position B5.degree. or B75.degree..
On the other hand, in a unstable operating range of the engine such as an early or initial stage of engine starting operation, the microcomputer 5 controls the ignition such that the power supply to an ignition coil of the engine is started at the second reference position B75.degree. and it is cut off at the first reference position B5.degree.. Such ignition control serves to not only ensure a relatively long power supply or conduction time enough to provide large discharge energy for firing a cylinder but also cause combustion of an air/fuel mixture therein at proper timing suitable for generating not less than a predetermined amount of torque required for the operating range of the engine, thereby ensuring a minimum engine operation.
In this case, however, since the camshaft is operatively connected with the crankshaft through a power transmission belt such as a timing belt, it is extremely difficult to always ensure that the camshaft rotates in exact synchronization with the rotation of the crankshaft. As a result, the reference position signal ST may involve a certain degree of error and thus does not exactly reflect or indicate the predetermined crank positions.
To avoid this problem, it has been proposed that the reference signal generator 2 be mounted on the crankshaft, but in this case, there are structural restrictions such as spatial problems for installation. Moreover, since the crankshaft performs two complete revolutions per one cycle of each cylinder, a single pulse of the reference position signal simultaneously corresponds two or more cylinders, thus making it difficult to perform identification of these cylinders. To this end, an additional cylinder identifying means is required.
In summary, with the conventional engine control apparatus as described above, the first and second reference positions B5.degree., B75.degree., which are detected or determined based solely on the reference position signal ST from the reference signal generator 2 mounted on the camshaft, involve more or less errors, so it is impossible to perform highly precise control on the engine using the first and second reference positions.